It is well known to measure the rotational position and the speed of rotation of shafts, and in particular that of a transmission shaft. This is often achieved through the use of a speed sensing arrangement.
U.S. Pat. No. 7,579,827 describes a speed sensing arrangement for sensing the speed of a transmission input shaft, which includes circumferentially spaced markings about the shaft and a speed sensor that is placed in close proximity to the markings The markings may be incorporated on a target wheel (also know as a speed sensor disc), which is attached to the input shaft. In one embodiment of the target wheel a gear ring is provided with a plurality of teeth that define the circumferentially spaced markings.
Speed sensor discs, such as the target wheel referred to above, are typically made from pressed steel plate, with serrations at the edge. The serrations allow the speed sensor to detect rotational movement of the shaft and thus determine how fast the transmission shaft is rotating. Pressed steel plate is generally used for the speed sensor disc in order to keep costs down. In order to fit the speed sensor disc amongst the other components on the transmission shaft, additional componentry, such as a thrust washer, is usually needed. Thrust washers are typically made from better quality steel than the speed sensor disc and are machined to provide a good surface finish. The thrust washer is required since the surface finish of the speed sensor disc is poor and this could potentially damage critical neighboring components on the shaft. It is also common practice to use a standard washer and bolt on the end of the shaft to ensure that the speed sensor disc is firmly held in place. Thus, such prior art speed sensing arrangements require four separate components to be mounted on the transmission shaft. This means that the shaft needs to be sufficiently long to provide space for the components to be mounted.
Furthermore these prior art speed sensors are mounted for radial measurement which means that the assembly must be configured to accommodate the speed sensor.